In recent years, environmental sensors have been used for safety management and maintenance of buildings, collection of environmental information, and the like. The environmental sensors are being improved in many aspects such as miniaturization and low power consumption.
One of the ways that have been proposed to achieve higher performance such as lower power consumption and miniaturization is the use of an oxide semiconductor for a semiconductor layer (hereinafter referred to as an active layer, a channel layer, or a channel formation region in some cases) of a transistor in a semiconductor device. For example, a transistor in which an oxide of indium, gallium, and zinc (hereinafter referred to as an In—Ga—Zn oxide in some cases) is used for a channel layer is given (see Patent Document 1).